I'm glad your piggys feeling better, but i think you also need to remind him of the Laws in this country..
First off remind him
1/Don't Be A Jerk!
2/If found guilty in a court of law, he can be fined between £1000 up to £25,000 (in extreme circumstances,) and between 6 months to 5 years in prison.
Heres the UK animal welfare act which i think he needs to read regarding all the laws to do with animals .
This is LAW your dad can't argue this, if he does I'm sorry that hes your dad!
http://www.statutelaw.gov.uk/content.aspx?activeTextDocId=2926439
scroll down to section 4 everything you need to know is right there, but the whole document is worth reading!
Prevention of harm
4
Unnecessary suffering
(1) A person commits an offence if—
(a) an act of his, or a failure of his to act, causes an animal to suffer,
(b) he knew, or ought reasonably to have known, that the act, or failure to act, would have that effect or be likely to do so,
(c) the animal is a protected animal, and
(d) the suffering is unnecessary.
(2) A person commits an offence if—
(a) he is responsible for an animal,
(b) an act, or failure to act, of another person causes the animal to suffer,
(c) he permitted that to happen or failed to take such steps (whether by way of supervising the other person or otherwise) as were reasonable in all the circumstances to prevent that happening, and
(d) the suffering is unnecessary.
(3) The considerations to which it is relevant to have regard when determining for the purposes of this section whether suffering is unnecessary include—
(a) whether the suffering could reasonably have been avoided or reduced;
(b) whether the conduct which caused the suffering was in compliance with any relevant enactment or any relevant provisions of a licence or code of practice issued under an enactment;
(c) whether the conduct which caused the suffering was for a legitimate purpose, such as—
(i) the purpose of benefiting the animal, or
(ii) the purpose of protecting a person, property or another animal;
(d) whether the suffering was proportionate to the purpose of the conduct concerned;
(e) whether the conduct concerned was in all the circumstances that of a reasonably competent and humane person.
(4) Nothing in this section applies to the destruction of an animal in an appropriate and humane manner.
I'm sorry to say but if your parents refuse treatment again phone 999 and ask for the police or the local RSPCA and explain the situation to them, both the police and RSPCA have the power to prosecute your parent for Neglect.
Your parents are very good as giving you a telling off but once in a while even parents need one, there not always right!
Hope this helps you and the well being of your little guys!